Ronaldo made his Champions League debut for Manchester United 16 years ago but failed to break his duck in four seasons until he bagged a brace in the famous 7-1 thrashing over AS Roma.
Twelve years on, he became the first player to reach the 125-goal mark with his thumping header against AFC Ajax in their 1-1 first-leg draw in Amsterdam within the quarter-final tie.
"Ronaldo showed that he is on a different level. His timing and movement is different to everyone else's - there's nothing you can do about it," Allegri told Sky Sport Italia.
"He is a player who just has a different technique than the others."
The 34-year-old has scored five goals and registered two assists in eight games in the competition this season.